Here is the situation: I provide services to a client through a company's platform. The company provides a software that keeps track of the amount of time I spend and automatically bills the client accordingly. Recently I found that due to a technical problem in the software, it was under-billing the amount of time and in the past few years, this has cost me thousands of $$. I talked to the company about it and they are now advising me to claim this money from the client directly trying to convince him about the issue.
I'm now writing to the company and want to express that he would find this claim deceitful. What would be the proper words to express my feeling? I'm considering words like underhanded, deceiving etc.
Also what would be the proper word or phrase for my feeling of being robbed and deprived of my money?